(1) Field of the Invention
This invention relates to a bumper core suitable for use in the fabrication of an automobile bumper and a process producing same.
(2) Description of the Prior Art
Metal-made bumpers have conventionally been used as automobile bumpers. Bumpers made of synthetic resins have been used due to their reduced weight in recent years, since weight reduction of automobiles saves energy. A bumper of the above sort, which is made of a synthetic resin, is generally formed of a foam core and a covering enclosing the core therein and made of a synthetic resin or the like. A foam core may consist of polyurethane foam, polyethylene foam, polystyrene foam or the like.
A foam core is an important part which governs the performance of the resulting bumper and is generally required to have excellent energy absorption capacity and dimensional predetermined recovery. Besides, cores having low densities fulfill the recent demand for lighter automobiles.
Polyurethane foams of the prior art cores for bumpers, are however accompanied by drawbacks such as to obtain the weight reduction due to their high densities (generally 0.09-0.15 g/cm.sup.3) and their production costs are high although they have excellent energy absorption capacity and dimensional percent recovery. Polyethylene foams have poor energy absorption capacity and heat resistance. The dimensional percent recovery and impact resistance of polystyrene foam are both poor. Prior art bumper cores have both merits and demerits as mentioned above but are unable to simultaneously satisfy three conditions required for bumper cores, namely, (1) excellent energy absorption capacity, (2) superb dimensional percent recovery and (3) low density for weight reduction.
As an improvement to the above-mentioned bumper cores, the present assignee had already developed as a result of a joint research with another party a bumper core formed of a molded article of foamed beads of a polypropylene-base resin and having a density and compressive stress respectively within specific ranges, described in a U.S. Pat. application Ser. No. 619,693 filed June 13, 1984 (which is a division of abandoned Ser. No. 504,289 filed June 14, 1983) now U.S. Pat. No. 4,504,534 dated Mar. 12, 1985. The above bumper core is an epochal one and satisfies the above-described three conditions for cores but lacks the dimensional accuracy desired.
Namely, the present bumper core is produced by the expansion molding process. After taking a molded article out of a mold, the molded article undergoes shrinkage although the shrinkage is small. This shrinkage however develops an error in the lengthwise dimension of the resultant core, whereby making it difficult to obtain molded articles of substantially identical dimensions. Thus, the above-developed bumper core has still some room for improvements with respect to its dimensional accuracy, because a bumper core is required to have particularly strict dimensional accuracy.